We have been to Napa at least once a year for years. Always stay with friends. My suggestion is book asap. Do some internet searching. It somewhat depends on where your niece is getting married. There are several nice places in the St. Helena area. There is Silverrado in Napa itself. In the past 5+ years there has been a lot of room expansion in the Napa Valley area so you should not have too much trouble, but book early.
Sorry could not give more specfic suggestions.
